Metering pumps of this type are known, particularly from EP-A-0,255,791 or from FR-B-2,707,350. These pumps are especially useful for injecting an additive, consisting of the auxiliary liquid, into the main liquid which serves at the same time as drive fluid. Such metering pumps have many uses. They make it possible, for example, to carry out a metering of chlorine (auxiliary liquid) into water (main liquid) or a metering of a drug for cattle into the water of a drinking trough or else to prepare beverages based on water (main liquid) and concentrate (fruit juice concentrate or tea concentrate) forming the auxiliary liquid.
For some uses, such as the metering of chlorine into water or of drugs for cattle into water, the proportion of auxiliary product to be added to the main liquid is low; it is relatively difficult to maintain this proportion exactly, particularly on account of leaks, albeit very slights, when a conventional seal is used, through which a rod slides, for example a connecting rod between the drive piston and the metering piston; such a seal does not make it possible to prevent tiny quantities of liquid on either side of the seal from being carried along due to capillary action.
Metering accuracy is therefore impaired by these, albeit very small, leaks.
In addition to the metering inaccuracy, these leaks eventually lead to various other problems, for example to scale being deposited on the rod, when chlorine is metered into hard water, or to the growth of bacteria, when a sweetened concentrated beverage (auxiliary liquid) is metered into water (main liquid).
These problems only become more serious during operation, since friction causes irreparable wear of the seal, which is accompanied by an increase in the passage of liquid on either side of the seal.